1. Field of the Invention
The present invention relates to an audio circuit for outputting sounds to the outside through a speaker for example.
2. Description of the Related Art
Recent personal computers are often provided with a display unit made of a liquid crystal panel. FIG. 2 illustrates a display unit of a typical desktop personal computer. The illustrated display unit 2 includes a liquid crystal display 3 arranged centrally at the front portion of the display unit 2. The display 3 is flanked by a pair of speakers 4a, 4b for outputting sounds to the outside. Generally, the speakers 4a, 4b are connected to an audio circuit (not apparent in FIG. 2) built in a housing of the display unit 2. The audio circuit feeds audio signals to the speakers 4a, 4b. Recently, the audio circuit generally utilizes a Hi-Fi system for providing high-fidelity sound to promote the so-called audio-visualization in the personal computers.
The liquid crystal display 3 generally utilizes a cold cathode ray tube as a back light source driven by an inverter. In controlling the lighting of the cold cathode ray tube by the inverter, pulse signals having a predetermined duty cycle are fed to the cold cathode ray tube. The brightness of the liquid crystal display 3 is adjusted by varying the duty cycle of the pulse signals.
As shown in FIG. 3, the audio circuit 1 includes a signal input 11 for inputting audio signals to a signal amplifier 12 for amplifying the audio signals. The signal amplifier 12 is connected to the speakers 4a, 4b. The signal amplifier 12 is connected to a ground line 6 to which an inverter 5 of the display unit 2 is connected.
In controlling the lighting of the cold cathode ray tube at the inverter 5, current fluctuates with an amplitude of about 1–2A in response to the pulse signals. Such current fluctuation causes noises to be fed to the signal amplifier 12 through the ground line 6. Therefore, noise-laden audio signals are outputted to the speakers 4a, 4b, which makes the listener feel unpleasant.